1. Field of the Invention
The present invention relates to a broadband antenna, and more particularly, to a miniature broadband antenna having high radiation efficiency and conforming to regulations of specific absorption rate.
2. Description of the Prior Art
As electronic products with wireless communication functionalities (e.g. tablet PCs, laptops, and mobile phones) become necessary tools in modern life, the number of wireless network applications is increasing, and the demand for higher transmission speed is getting stronger. Broadband antennas are therefore in great demand, especially to comply with advanced communication protocols such as the Long Term Evolution (LTE) technology. Generally, one needs to design a larger antenna in order to obtain broader bandwidth. However, the antenna dimensions need to be minimized to meet the goal of producing thinner and lighter products.
The specific absorption rate (SAR) is one of the essential considerations for antenna designs. In order to conform to regulations of SAR, one should avoid designing a 3D antenna for mobile devices. However, designing a planar antenna does not guarantee that the antenna can pass the SAR criteria. Therefore, it is quite challenging to design an antenna having good radiation efficiency, broad operating bandwidth, small size, and also conforming to the regulations of SAR.
Common types of broadband planar antenna suitable for operating in LTE frequency bands are the planar inverted-F antennas (PIFA) and monopole/parasitic-part combined coupling antennas. The planar inverted-F antennas have conductive pins which may help to improve impedance matching. However, they require larger space to achieve broader bandwidth and better radiation efficiency. The coupling antennas usually have smaller dimensions. However, their performance may be easily affected by environment, and they are hard to be designed with matched impedance.
On the other hand, loop antennas are relatively easy to conform to the regulations of SAR; however, the antenna dimensions are larger since the lengths of their radiating elements should be as long as half wavelength of the resonant frequency. Moreover, their input impedance is too high to be adjusted easily, which therefore narrows the operational frequency bandwidth. As a result, conventional loop antennas are unable to cover all of the frequency bands for LTE applications. Loop antennas are usually used for applications operating in very high frequency bands (e.g. millimeter-wave frequencies), but not for applications operating in LTE frequency bands.
Therefore, it has become a common goal in the industry to design an antenna with reduced antenna dimensions and improved antenna bandwidth while the antenna maintains good radiation efficiency and conforms to the regulations of SAR.